(1) Field of the Invention
The present invention relates to signal processing means for providing improved turnaround time stability and more particularly to a means for improving the turnaround time stability of certain R-C energy detector based signal processors with respect to doppler and signal to noise ratio effects while minimizing the false alarm rate.
(2) Description of the Prior Art
The present invention is used in conjunction with a signal processing subsystem of an energy detector system. The subsystem employs an R-C integrator type energy detector for recognition of the presence of electrical signals and for timing of subsequent actions which are initiated upon receipt of such signals. In order to illustrate a typical energy detector or system and a signal processing subsystem, an exemplary prior art acoustic transponder system is described below.
Acoustic transponder system 10 is presented in block diagram form in FIG. 1. In use, one or more of these transponders 10 are tethered to the ocean floor in a preselected configuration as part of an acoustic tracking system. This acoustic tracking system also includes a plurality of separate acoustic receivers arranged in a preselected pattern, the output of which is used by general purpose digital computers for range and bearing determinations. A vessel being monitored by the tracking system transmits acoustic pings at regular intervals, which pings are received at one or more acoustic transponder systems 10. Each acoustic ping is converted by the transducer section 12 of transponder system 10 into an analog electrical ping signal 14 which is then transmitted to and through a transmit/receive network 16, emerging as ping signal 18. Ping signal 18 is transmitted to signal processor 20 which separates the ping from interfering background disturbances and provides as output a valid digital ping timing signal 22. At some constant preselected time after the valid ping is recognized by signal processor 20, a digital timing signal is sent to a transmitter circuit via control logic circuit 24, and output connections 26 and 28. This timing signal causes transmitter circuit 30 to initiate a ping transmission signal 32 via network 16 to acoustic transducer 12 which projects a ping back into the water. Transmit/receive switch network 16 senses the signal flow direction and switches acoustic transducer 12 into a receiver mode or a projector mode as appropriate. Output connection 28 from control logic 24 divides into output 28a to signal processor 20 and output 28b to transmitter circuit 30. Outputs 28a and 28b serve to lockout device 20 while device 30 is operating and vice versa.
Thus, each successfully recognized "valid" ping causes transponder system 10 to reply with a corresponding ping to at least one separate acoustic receiver in the tracking system. The precise time at which each ping is transmitted by the vessel being tracked is carefully controlled by the tracking system. Range information is then deduced from transmit time, range geometry and the reply ping arrival time using a computer. Any uncertainty introduced in this arrival reply ping time will directly translate into uncertainties in the tracking solution. As used herein "uncertainty" refers to the timing reference period which ideally should be held constant.
A typical signal processor 20 used in transponder system 10 is illustrated in FIG. 2. Signal processor 20 further comprises a lockout switch 34 which is controlled by output 28a from control logic unit 24 of FIG. 1. When transponder system 10 is in receive mode, switch 34 is closed permitting received electrical ping signal 18 pass into processor 20 for signal conditioning in amplifier/clipper 40, output 42 of which is filtered by wideband bandpass filter 44. The filtered output signal 46 from filter 44 is next passed through a second amplifier/clipper 48, emerging as signal 50. Signal 50 is then fed to narrowband filter 52, emerging as modulated ping signal 54. Modulated ping signal 54 is demodulated by R-C energy detector 56 and as demodulated ping signal 58 is inputed to validation threshold adjustor 60, adjustor 60 being set, as a voltage divider referenced to ground 62. This assures that an amplitude large enough to be a valid ping will trigger one shot 64 which produces a digital timing pulse 22 indicative of a valid ping signal having been received.
FIG. 3 graphically depicts the timing signals for the processor of FIG. 2, the component numbers thereof being used in conjunction with the FIG. 3 discussion. Each received electrical ping signal, such as that shown in FIG. 3A, is a short continuous wave (CW) burst of constant amplitude and frequency. The ping signal energy is proportional to [Voltage (V)].sup.2 .times.[time (t)]. Normally this signal is embedded in background interference and exhibits a shifted center frequency due to the speed of the tracked vessel, i.e., doppler shift. This signal is amplified/clipped, 40, and then applied to wideband bandpass filter 44 to remove some of the background interference. The signal is next amplified/clipped again, 48, and applied to narrowband bandpass filter 52. Enough of the interference is removed at this point to enable the signal to be recognized by R-C energy detector 56. Typically an R-C energy detector comprises a rectifier and an R-C integrator. Energy from the rectified ping signal charges the integrator capacitor (C) through a resistor (R) resulting in the demodulated ping signal shown in FIG. 3B. When the demodulated ping signal exceeds a preset threshold amplitude, one shot 64 is triggered causing the valid ping timing signal to be issued to transmitter circuit 30 previously described. The signal processor "turnaround time" shown in FIG. 3C represents the time interval between the beginning of the received electrical ping signal and the digital timing reference trailing edge of the valid ping timing signal from one shot 64. The valid ping timing signal pulse itself is a constant preset one shot time period. Uncertainties in the variable portion of the turnaround time, from the beginning of the received electrical signal to the one shot threshold, directly affect the reply ping arrival times at the separate acoustic receivers and thus introduce errors into the range tracking solution for the vessel.
Contradictory considerations effect the one shot threshold settings shown in FIGS. 3B and 3D. Since some interference always remains embedded in the demodulated ping signal, it is desirable to raise the threshold to a voltage level sufficient to prevent the interference from producing a valid ping timing signal in the absence of a true ping, i.e., a false alarm. However, it is also desirable to set this threshold voltage level as low as possible to minimize the turnaround time uncertainty caused by differences in the demodulated ping signal amplitudes of different pings as shown in FIG. 3D. Corrupted and uncorrupted demodulated ping signals are shown to illustrate the effect of curve slope on the time measurements at constant threshold voltage settings Threshold 1 and Threshold 2 of FIG. 3D. These time differences are caused by varying degrees of amplitude signal-to-noise ratio and doppler shift as the vessel moves about the range. FIGS. 3D-3F illustrate the latter signal variation condition. Threshold 2 corresponds to a higher voltage and hence an optimum false alarm rate setting while Threshold 1 corresponds to a shorter time slice at more favorable segment of the amplitude curve for timing and hence a reduced turnaround time uncertainty, t. Turnaround time uncertainty versus demodulated ping signal amplitude for the conditions of threshold 1 and threshold 2 is illustrated in FIG. 3F. The optimum uncertainty would be zero, but the smaller the t is the better. Setting threshold 1 lower takes advantage of the steeper slope portion of each successive ping curve.